The design of a commercially competitive fuel injector normally involves acceptance of some characteristics which are less than optimal since the basic goals of low cost, high performance and reliability are often in direct conflict. For example, cam operated unit injectors, such as disclosed in U.S. Pat. No. 3,544,008, are more expensive to construct but are more reliable and accurate than are distributor-type fuel injector systems having a single centralized high pressure pump and a distributor valve for metering and timing fuel flow from the pump to each of a plurality of injection nozzles as disclosed in U.S. Pat. No. 3,557,765.
As the need for higher engine efficiency and pollution abatement have increased, it has become increasingly evident that some economical means must be provided to have constant injector timing in response to changing engine operation conditions. Such control is relatively straight forward in distributor-type fuel injector systems since the injection event is controlled at one central location. However, in unit injector systems, providing constant injector timing while varying the metered quantity of fuel ordinarily requires modification of each individual unit injector, thereby adding significantly to the overall cost of the system.
U.S. Pat. Nos. 2,997,994 and 2,863,438 provide examples of attempts to solve this dilemma. In particular, these patents disclose the use of a collapsible hydraulic link to selectively change the effective length of the cam operated fuel injector plunger. However, the simplicity of these hydraulic timing controls is only achieved by operating the hydraulic link in either a fully expanded or fully collapsed mode. Thus there is a stepped change in timing of the injection event which will not necessarily suit the broad range of conditions normally encountered during the operation of an engine. Attempts to provide for variations in the start of injection, even when a hydraulic link is employed, have generally involved the use of a mechanical rack which controls the size and/or the point of collapse of the hydraulic link. Examples of such hydraulic/mechanical systems are disclosed in U.S. Pat. Nos. 3,847,510 and 4,092,964.
Hydraulic arrangements are often used for providing infinite variation of unit injector timing as illustrated in U.S. Pat. Nos. 3,035,523 and 3,083,912 wherein fairly complex hydraulic arrangements for this purpose are disclosed. However, in these systems the quantity injected and the change in timing are interrelated and may not be controlled independently of one another. Moreover, the timing of injection varies with each stroke.
Other types of injectors employing a hydraulic link which may effect injector timing have been disclosed such as in Danish Patent No. 56,902 issued Nov. 6, 1939 and U.S. Pat. Nos. 3,029,737 and 3,782,864. However, these additional disclosures do not teach how to achieve constant start of injection while varying the quantity of fuel injected. Similarly, U.S. Pat. No. 4,463,901 discloses the use of a hydraulic link for relating the timing and metering chambers to one another, however, again this disclosure does not utilize such linkage to achieve constant start of injection while varying the quantity of fuel injected.
Presently, fuel injection systems for achieving constant start of injection utilize various valving arrangements or distributor configurations. However, due to time delays and pressure waves in the fuel lines, the start of injection retards with engine speed.
In short, the prior art fails to disclose a low cost, highly reliable fuel injector which provides constant start of injection while varying the metered quantity of fuel injected.